An electron stereoscan microscope is used to examine the surfaces of an aluminum cathode and an aluminium anode which have been subjected to repeated sparkings, under an ultrahigh vacuum (10−9Torr), at voltages of up to 45 kV. The cathode surface is observed to contain large numbers of protrusions which give rise to an enhanced local electric field at the tip of the protrusions. The field enhancement factor, &bgr;, at the microprojections is determined from the scanning electron micrographs. The values of &bgr; are in resonable agreement with those obtained from the Fowler‐Nordheim theory when applied to the current‐voltage measurements in the region prior to breakdown. The anode surface is completely devoid of protrusions and contains large numbers of craters. The diameters and the depths of typical craters are also determined.